In a piece for EJIL:Talk! Blog of the European Journal of International Law, CERL Advisory Council member Mary Ellen O’Connell uses the case of 9/11 to examine the legality of launching a ground invasion after an act of terrorism. She argues that even when Article 51 of the United Nations Charter permits the use of force in self-defense, the defending state must comply with the principles of necessity and proportionality.

Mary Ellen O’Connell is the Robert and Marion Short Professor of Law and Research Professor of International Dispute Resolution—Kroc Institute for International Peace Studies, University of Notre Dame. Read her bio here.

The views expressed here are the author’s own and do not necessarily represent those of any organization or university.